1. Field of the Invention
This invention relates to methods of and apparatus for digital video signal processing.
2. Description of the Prior Art
Various coding standards have been adopted for digitally encoding the video component of a television signal. One coding standard in common use is the 4:2:2 CCIR 601 standard in which a luminance component is sampled at 13.5 MHz and each of two chrominance components C.sub.B and C.sub.R is sampled at 6.75 MHz. Digital video tape recorders (DVTRs) for recording and reproducing 4:2:2 coded signals with high fidelity are now in common use.
A higher quality standard in the same family, which is beginning to come into use for studio reference signals, is the 4:4:4:4 (or 4.times.4) system in which a luminance component and each of two chrominance components C.sub.B and C.sub.R are sampled at 13.5 MHz, as is a fourth component, which Is a full bandwidth linear key signal used, for example, in some special effects. There is a requirement to separate or demultiplex a 4:4:4:4 signal into two 4:2:2 signals so that it can be recorded using two 4:2:2 DVTRs, and after subsequent reproduction can be reconstituted. It is very desirable to achieve the demultiplexing in such a way that at least one of the 4:2:2 signals is a true CCIR 601 standard signal suitable for direct use with other CCIR 601 standard equipment.
Problems arise in effecting such separation, particularly with the chrominance components.
Simple demultiplexing of the 4:4:4:4 signal into two 4:2:2 signals is not acceptable, because the chrominance components in each of the resulting 4:2:2 signals will contain large amounts of alias frequencies. Nor is simple frequency separation into low frequency and demodulated high frequency parts acceptable, because there will be filter cross-over regions which will cause significant errors in the region of 3.375 MHz.